This invention relates to a method of recording image with ink droplets and ink mists. The invention also relates to a printer using the method.
Various types of ink-jet printer have been proposed to date and they share the common feature of ejecting ink droplets for dotting to paper or the like to record image. In recent years, the array density of ink nozzles on the recording head of the ink-jet printer has been increased to provide higher resolution and this has contributed to producing a sharp image having a comparatively large number of tone levels of gradation. On the other hand, the volume of each ink droplet that is ejected is still large, generally at least one picoliter (pL), so with the increase of the array density (dot density) of ink droplets ejected, it takes more time to dry the paper or the like on which image has been recorded by densely deposited ink droplets and this makes the ink-jet printer unsuitable for high-speed recording (printing).
It has been proposed to equip a printer with a preheating means that heats the paper or the like prior to recording so that the deposited ink can be dried in a short enough time but this has not necessarily attained the desired result. In order to meet the future demands for higher dot density of ink droplets and faster printing, it is essential to realize rapid drying of the recording paper or the like.
Very recently, it has been proposed to record image by jetting out ink mists in an amount of about 0.1 pL against paper or the like (Hiroshi Fukumoto et al. xe2x80x9cMulti-gradation Ink-Jet Recording Using Mists jetted out by Converging Ultrasonic Wavesxe2x80x9d, ""99 Proceedings of Japan Hard Copy, 1999, pp. 343-346). In this method, ink mists each of which consists of a mass of fine ink drops are applied to record dots, thereby controlling the gradation density of each dot to be represented in multiple tone levels of gradation. It is said that the use of ink mists rather than ink droplets permits image gradation to be represented smoothly enough to achieve a substantial improvement in image granularity.
Since jetted ink mists are deposited on paper or the like in amounts of about 0.1 pL, the drying of ink would be fast enough to permit high-speed printing. On the other hand, the fine ink drops of an ink mist are not controlled individually but a plurality of the fine ink drops are simultaneously jetted out to pour like a shower until they are deposited in a specified area of paper or the like; hence, the edges of line images or those of a solid tone level of gradation cannot be reproduced sharply.
An object, therefore, of the present invention is to provide a method which not only permits fast drying of ink to realize high-speed printing but also enables the edge portions of a solid tone level of gradation in a image to be recorded sharply.
Another object of the invention is to provide a printer using the method.
These objects may be achieved, in the first aspect of invention, by providing an image recording method for recording an image having edge portions of a solid tone level of gradation therein, having steps of; recording the edge portions with dots to form a solid tone; recording portions in the image other than the edge portions using ink mists that are generated by converging ultrasonic waves in ink.
Preferably, the edge portions are recorded using ink droplets.
The solid tone is a tone which is formed by dotting without any space left for dotting on image recording to obtain the highest density that dots can represent.
The image includes not only an image having multiple tone levels of gradation such as photographed image, but also an image composed of lines, characters and their combinations which have a single solid tone level.
In the second aspect, the invention provides a printer having a recording head which ejects ink droplets or jets out ink mists selectively, depending upon input data of an image.
The printer preferably has a drive signal generating section which, depending upon the input data, generates a first drive signal for causing the recording head to eject ink droplets or a second drive signal for causing the recording head to jet out ink mists and the drive signal generating section applies either the first drive signal or the second drive signal to the recording head.
Additionally, the drive signal generating section preferably generates the first drive signal for edge portions of a solid tone level of gradation in the image to be recorded and generates the second drive signal for portions other than the edge portions.
Then the first drive signal preferably has lower frequency than the second drive signal.
Preferably, the recording head has liquid ink chambers each of which has an ink nozzle through which ink droplets are ejected or ink mists are jetted out selectively, and signal values of the first drive signal and the second drive signal are controlled such that a liquid level of ink in each liquid ink chamber is brought into registry with an outlet surface of the ink nozzle when the second drive signal is applied to the recording head whereas the liquid level in the liquid ink chamber is made lower than the outlet surface of each ink nozzle when the first drive signal is applied to the recording head.
Additionally, the edge portions are preferably detected on pixel basis of the image to find pixels which differ from adjacent pixels by tone level of gradation exceeding a specified value.
In the third aspect, the invention provides a printer having at least two recording heads, a first being a recording head which records edge portions of a solid tone level of gradation in an image to be recorded with dots to form a solid tone, and a second being a recording head which records portions in the image other than the edge portions using ink mists that are generated by converging ultrasonic waves in ink.
Preferably, the edge portions are detected on pixel basis of the image to find pixels which differ from adjacent pixels by tone level of gradation exceeding a specified value.